Aircraft engine noise is a significant problem in high population areas and other noise controlled environments. Attempts currently focus on lining the aircraft engine nacelle and surrounding engine areas with acoustic liners to reduce the amount of noise radiating to the community.
As background information regarding general engine acoustic theory, there exists a linearized wave equation that describes the acoustic pressure distributions present in an airflow duct. This wave equation has a general solution given by a superposition (i.e., discrete summation) of eigenfunctions. Eigenfunctions vary with the boundary conditions at the duct wall, i.e., the wall's impedance. There are an infinite number of such eigenfunctions, each with an associated eigenvalue, that are referred to as the "modes of propagation", or "modes" for short. In general, low order modes have eigenvalues that are low in absolute value. High order modes have eigenvalues that are high in absolute value. Typical low mode order values for aircraft engine noise are 0 to 5, though the range will change depending on frequency, duct size, etc. Typical high mode order values for aircraft engine noise are 8 to 15, though, these will also vary.
As used in the discussions below, the term "low mode order noise" is meant to describe noise waves that are represented mathematically by relatively low absolute value eigenvalues with respect to the range of noise modes present in a given application. When viewed physically, the order of mode corresponds generally to the angle of wave propagation in the duct relative to the duct walls. As shown in FIG. 1, engine noise wavefronts 12 propagate along the duct 13 at various angles .theta. relative to the duct walls 14. If the angle is zero, the wave is said to be a fundamental wave 15, i.e., .theta.=.theta..sub.f =0. Fundamental waves have wavefronts that travel in the axial direction of the duct and have a uniform pressure distribution at any particular duct cross-section.
In addition to fundamental waves, there are non-fundamental noise wavefronts which reflect back and forth between the duct walls as the wavefronts travel along the duct. These non-fundamental wavefronts create a non-uniform pressure distribution across the duct cross-section. The non-fundamental waves are generally classified according to their angular directions relative to the duct walls. Low order modes of noise propagation have wavefronts 16 oriented at small angles as measured relative to the duct walls, i.e., .theta.=.theta..sub.1.ltoreq. approximately 30 degrees. High order modes of noise propagation have wavefronts 17 oriented at relatively larger angles as measured from the duct walls, i.e., .theta.=.theta..sub.h.gtoreq. approximately 60 degrees. A wide range of noise frequencies exists for each mode order, low or high. For further discussion of theoretical considerations, see Aeroacoustics of Flight Vehicles, by Harvey H. Hubbard, published for the Acoustical Society of America through the American Institute of Physics, 1995. See also, Theoretical Acoustics, by Philip M. Morse et al., McGraw-Hill Book Company, dated 1968.
In a given aircraft application, an engine will generate both high and low mode order noise. Current design practice focuses on reducing this noise through the use of absorptive acoustic liners. Absorptive liners are known in various configurations, including the use of a honeycomb core sandwiched between an imperforate sheet and a perforate sheet having a small amount of open surface area. This particular combination is sometimes referred to as a single degree of freedom absorptive acoustic liner.
Absorptive liners are successful because pressure waves cause air to pass into and out of the openings of the perforate sheet and to experience a sufficient amount of friction, or resistance, which is dissipated as heat energy. The overall impedance of an acoustic liner is a complex number, given by a real part, the resistance, and an imaginary part, the reactance. Resistance relates to the liner's ability to dissipate noise energy as heat. Reactance relates to the liner's tendency to react noise energy back onto itself. Absorptive liners provide moderate resistance and low reactance for high mode order noise waves.
FIG. 2 illustrates the theoretical effect of using absorptive liners for a hypothetical case. A given total noise energy 18 is initially comprised of a combination of one low and one high mode order noise 19, 21, each having equal energy. Starting at the beginning of the duct at position 0, the total noise energy 18 encounters an absorptive liner that quickly reduces the high mode order noise 21 and more slowly reduces the low mode order noise 19. Since high mode order noise attenuates quickly in the duct, only a relatively short duct length is needed to dissipate most of the high mode order noise present. In FIGS. 2 and 3, the vertical axis is logarithmic. A change of about -3 dB, for example, refers to a reduction in noise energy of about half. The horizontal axis is normalized to be dimensionless. The exact values of the information shown in FIGS. 2 and 3 will vary according to the characteristics of a particular application, and in general there will be energy in more than two modes.
As is evident by FIG. 2, absorptive liners are very effective for absorbing high mode order noise 21, but are inefficient for reducing low mode order noise 19, i.e., those noise wavefronts traveling along the duct at a low angular displacement relative to the duct walls. Propagating at low angles, these low order modes strike the absorptive liners fewer times in a given length of duct. Therefore, to reduce all of the low mode order noise requires a greater length of acoustic lining than is typically possible in the space-limited regions of aircraft engines. Noise reduction from use of absorptive liners is thus practically limited to higher mode order noise.
Thus, a need exists for an acoustic liner, or arrangement of liners, that effectively reduces both high and low mode order noise. The present invention is directed to fulfilling this need.